Bulk density monoliths

From isotherm measurements, usually earried out on small quantities of adsorbent, the methane uptake per unit mass of adsorbent is obtained. Sinee storage in a fixed volnme is dependent on the uptake per unit volume of adsorbent and not on the uptake per unit mass of adsorbent, it is neeessary to eonvert the mass uptake to a volume uptake. In this way an estimate of the possible storage capacity of an adsorbent can be made. To do this, the mass uptake has to be multiplied by the density of the adsorbent. Ihis density, for a powdered or granular material, should be the packing (bulk) density of the adsorbent, or the piece density if the adsorbent is in the form of a monolith. Thus a carbon adsorbent which adsorbs 150 mg methane per gram at 3.5 MPa and has a packed density of 0.50 g/ml, would store 75 g methane per liter plus any methane which is in the gas phase in the void or macropore volume. This can be multiplied by 1.5 to convert to the more popular unit, V/V. 

A wide range of polymeric materials can be prepared from HIPEs. Polymerisation of the continuous phase yields highly porous cellular polymers with a monolithic structure. These are known as PolyHIPE polymers, and possess a number of unique properties including, in most cases, an interconnected cellular structure and a very low dry-bulk density. Their very high porosity favours their use as supports for catalytic species, precursors for porous carbons and inert matrices for the immobilisation of enzymes and micro-organisms. 

The bulk density of materials was measured by Hg pycnometry from independent measurements of the mass and the volume of monolithic samples. The geometrical volume of the sample is determined fi om the weight difiference between a flask (calibrated volume) filled up with mercmy and the same flask filled up with the sample and mercury. As mercury is a non-wetting liquid and as no pressure is exerted, mercury does not enter in the porosity of the sample or crush it. 

The two promising candidates are adsorbent monoliths and adsorbent sheets. The fabrication of activated carbon and zeolite monoliths are reported in the literature. Zeolite monoliths have also been tested for air separation application by PSA.50 51 However, the use of monoliths for use in H2 PSA is not known to the authors. Monoliths having very high cell density (several hundred to thousand cells per square inch) will be necessary in order to have fast adsorption kinetics as well as reasonable bulk density for a PSA application. Manufacture of such monoliths is complex, and they are not yet commercially available. Gas channeling through the monoliths can also be a problem.52 Adsorbent sheets have been used for air separation by RPSA.53 54 The thickness of the adsorbent sheets and the space between the... 

The Amoco PX 21 carbon has the highest BET surface area of aU carbons listed in Table 22.3. Yet, the isothermal deliverable CH4 capacity for this carbon is about lOOm /m [18]. This is due to its low bulk density (high external void). If this carbon can be produced in a monolith form so that the external void in the ANG tank is negligible, then the isothermal deliverable capacity ( 210m /m ) approaches that of the CNG [18]. Recendy, a carbon monolith was fabricated by Lozano-CasteUo and coworkers, but it produced an isothermal deliverable capacity of 126m /m only [40]. 

A variety of monoliths are offered to the market which differ in the size of the channels and in the thickness of the walls. Together, these parameters fix what is called the cell density, the number of cells per unit frontal surface area. With ceramic monoliths, these two parameters can be varied to some extent independently. As of today, the most commonly used supports have about 62 channels per square centimeter, which corresponds to a channel width of about 1 mm and a wall thickness of about 0.15 mm. The bulk density of such a support is about 420kgm . The supports currently used have, within the production variance, homogeneous channel... 

These parameters, together with the porosity of the monolith walls, also affect the bulk density of the monolith, and therefore both the weight and the thermal mass of the converter. However, the data reported in the literature indicate that the influence of these parameters on the conversion reached over the catalyst is rather small, especially when considering the performance of aged catalysts. 

On the other hand, the mechanical properties of monolithic carbon gels are of importance when they are to be used as adsorbents and catalyst supports in fixed-bed reactors, since they must resist the weight of the bed and the stress produced by its vibrations or movements. A few smdies have been published on the mechanical properties of resorcinol-formaldehyde carbon gels under compression [7,36,37]. The compressive stress-strain curves of carbon aerogels are typical of brittle materials. The elastic modulus and compressive strength depend largely on the network connectivity and therefore on the bulk density, which in turn depends on the porosity, mainly the meso- and macroporosity. These mechanical properties show a power-law density dependence with an exponent close to 2, which is typical of open-cell foams. 

Monolithic, dense and crack-free SiAlOC ceramic components were obtained (see Fig. 6) after a pyrolysis process (l°Cmin", Ar flow, 100cm min ) at 1000°C for 1 h. The weight and the length of the samples were recorded before and after the heat treatment in order to measure the associated weight loss and the linear shrinkage. The microstructure of the ceramic samples was investigated measuring the open porosity, F, and the bulk density, pt,. 

The space time values are calculated by the authors based on the information presented in the source articles. Most of the studies did not report the catalyst porosity and the bulk density, therefore the space time units had to be determined in the gcat.min/cm units. These units make it difficult for using the resulting kinetic information in monoliths where it is more meaningful to use proper time units for the space time due to the open geometry used in the monoliths. The adaptation procedure will be discussed in the forthcoming section. 

Catalyzed ceramic foam as a catalytic diesel particulate filter is an application different from other tems discussed in this paper. Here high temperattoe resistance and low bulk density are important, but tortuous flow paths together with open porosity are key features. As with conventional monolithic catdysts, cordierite foams have been used because of the very low thermal coefficient of e q)ansion [40-45]... 

Smith et al. [14] had reported that the porosities of porous and hardened Ebonex were 12-15 % and <2 %, respectively, and that the voltammetric behavior of Ebonex in terms, for example, of the potential range of water stability is highly dependent on porosity [9]. Recently, Kitada et al. [15] reported selective preparation of macroporous monoliths of Magneli phases having widely different porosities and bulk densities. 

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